The world’s biggest investment banks saw combined revenues sink by 15 per cent in the first half of this year, the most since the aftermath of the financial crisis, underlining the urgency of taking radical measures to boost returns to shareholders.The second quarter saw more activity than the first, where there were simultaneous falls across stocks, bonds and commodities and a dearth of companies doing deals. But the rebound was not strong enough to avert a slump in total revenues from investment banking for the top 12 banks, according to figures from Coalition, a London-based group that combines public information with independent research and validation by a network of market participants.